1. Field of the Invention
The present invention relates to a magnetic bearing apparatus and more particularly to a magnetic bearing apparatus capable of being used under a clean atmosphere, for example, a semiconductor fabrication process, and capable of reducing in size in the rotational axis thereof.
2. Description of the Related Art
At present, a ball bearing is mainly employed as a bearing member for supporting a rotor in a rotational manner, such as a chemical vapor deposition (CVD) apparatus for growing a thin film of a predetermined component over a semiconductor substrate or a wafer.
Such chemical vapor deposition apparatus needs to be run in an evacuated clean atmosphere. In the current apparatus that uses the ball bearing as its component, however, a lubricating oil of the ball bearing will diffuse to contaminate the thin film to be formed on the wafer, thereby lowering the production yield. Further, rust on the ball bearing causes not only contamination in the clean atmosphere, but also results in imperfections on the outer surface of the ball bearing itself, thereby making the stable rotation thereof impossible. In this case, the ball bearing has to be replaced with a new one. However, this replacement adds time and cost in the fabrication process.
Therefore, investigations have been made to employ a magnetic bearing as a component in place of the ball bearing. The magnetic bearing has been developed in various researches and applications in numerous aspects due to non-contact, no lubrication requirements, and long lifetime characteristics.
A current magnetic bearing apparatus is constituted, for example, as shown in FIG. 1. This magnetic bearing apparatus 90 includes: a cylindrical rotor 93 accommodated in a casing 91 and equipped with a thrust disk 93a at the lower end side of a rotational axis 92, and electromagnets 94a and 94b constituting a thrust magnetic bearing 94 for supporting the rotor 93 in a non-contact manner in the thrust direction by making their magnetic poles face (or confront) each other through gaps in the upper and lower surfaces of the thrust disk 93a. The rotor 93 is supported in a non-contact manner in the radial direction by radial magnetic bearings 95 and 96 which are disposed at the upper portion and the lower portion of the rotational axis 92, and is rotationally driven on the rotational axis 92 by an electric motor 97.
The radial magnetic bearings 95 and 96 include: cylindrical laminated yoke portions 93b and 93c fixed on the rotor 93 such that they are laminated in the direction of the rotational axis 92, and upper electromagnets 95a, 95b, 95c and 95d (the electromagnets 95b and 95d are not shown) and lower electromagnets 96a, 96b, 96c and 96d (the electromagnets 96b and 96d are not shown) fixed on the casing 91 at positions to face (or confront) the yoke portions 93b and 93c and are proportionally divided into four segments (quadri-divided) in the circumferential direction.
Moreover, displacement sensors (not shown) for detecting the axial and radial displacements of the rotor 93 is disposed in the casing 91 so that the thrust magnetic bearing 94 and the radial magnetic bearings 95 and 96 are controlled by the control signals produced in a control unit (not shown) in accordance with the detected displacements.
In the magnetic bearing apparatus as constituted above, the thrust magnetic bearing 94, the radial magnetic bearings 95 and 96, the displacement sensors (not shown) and the electric motor 97 are disposed along the direction of the rotating axis 92. This construction elongates the magnetic bearing apparatus in the direction of the rotational axis 92. Because the laminated yoke portions 93b and 93c in the rotor 93 are exposed at their laminated end surfaces to the inside of the evacuated casing 91, rust, if allowed to grow due to environmental circumstances, will be scattered by the rotations of the rotor 93 to contaminate the inside of the casing 91. In the CVD apparatus, especially, the thin film to be formed over the wafer may be contaminated, resulting in a lower production yield.
The magnetic bearing apparatus thus described is elongated in the direction of the rotational axis because the radial magnetic bearings are disposed at a plurality of positions along the axial direction. Further, rust on the end surfaces exposed to the inside of the casing of the laminated yokes may contaminate the inside of the casing.